Compositions of the general type indicated above, i.e., wherein high permittivity is sought, are known, for example, from U.S. Pat. Nos. 3,258,522 and 3,287,489, along with British Pat. No. 1,394,272, all of which disclose the use of carbon black in the development of high permittivity elastomeric compositions. Further, the use of ceramic or high permittivity inorganic fillers in compositions is disclosed in U.S. Pat. Nos. 3,585,274; 3,673,305; 3,816,639; 3,823,334; 3,828,115; and 4,053,702. The combination of carbon black and planarly oriented conductive platelets in highly polar organic polymer bases, in the form of thin films, has been shown to provide high permittivity with high dielectric strength, as is disclosed in U.S. Pat. No. 3,349,164. Other known configurations include multi-layer heat shrink products which consist of a low permittivity heat shrinkable polymeric cover which has been internally coated with an elastomeric layer which provides high permittivity principally through the incorporation of silicone carbide particles, although conducting particulate fillers may also be included: see for example U.S. Pat. No. 3,950,604. Combinations of electrically conducting and insulating flakes are also known, as is disclosed in U.S. Pat. No. 4,234,439.
The usefulness of inorganic particulate materials such as barium titanate and titanium dioxide as high permittivity components for compositions having or exhibiting a refractive stress controlling action has been known for a long time, but the desirable elastomeric properties were overlooked; see for example U.S. Pat. Nos. 3,673,305 and 3,823,334. In this connection, it has been found that when inorganic materials of very high permittivity are utilized, e.g., barium titanate having a permittivity of approximately 6,000 to 10,000, the permittivity of the elastomeric composition cannot be increased beyond approximately 20 if the composition is to retain the desirable elastomeric properties of the base material. In other words, in order to provide satisfactory permittivity, the proportion of inorganic material becomes so high as to preclude the manufacture of practical stress control elements having adequate elastomeric characteristics for providing gap-free contact with electric power cables.
This invention provides high permittivity stress control for high voltage power cable terminations with greatly improved impulse strength. Furthermore, the mechanical strength and high elastic memory are sufficient for the pre-stretched tube delivery during application.
There are other stress controlling devices which contain materials mainly acting in a resistive manner. Such materials have the inherent disadvantage that an increase in resistivity under overload conditions may result in excessive heating and accellerated aging of the material, which can ultimately lead to electrical breakdown. Conversely, this invention acts, together with the cable insulating materials of low permittivity, upon electrical fields in the sense of refraction.
The application of pre-stretched tubular high permittivity stress control elements requires less knowledge and skill than the application of other stress control devices, such as the mounting of conductive stress control cones, the wrapping of stress control tapes, the molding of flowable or shapeable masses having stress controlling properties which subsequently harden at the site of installation, etc. Tubular termination designs also provide favorable economics to both the supplier and user, as less material is utilized, ultimate diameters are smaller, and creepage lengths between conductor and ground are shortened, thereby reducing space requirements at the installation site.